YOLOv3 Model Structure: Why is filters = (classes + coords + 1) * num?
As said by @brale in the comment below the question:

filters = (classes + 5) * 3
= (classes + width + height + x + y + confidence) * num
= (classes + 1+1+1+1+1) * num
= (classes + 5) * num


YOLOv3 dectects 3 boxes per grid cell, so it is:

filters = (classes + 5) * 3